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View Full Version : A few questions to start a DIY mini CNC



purple_rob
15-10-2008, 05:48 PM
Hello all,

I only thought about building a mini cnc last night and have only spent about an hour looking around the net at ideas.

I did'nt really get anywhere but have a few ideas and thought i'd post a few questions to see if what I want to achieve is even possible.

I'm looking at building quite a small CNC to enable me to machine materials such as: hard resin, nylon and possible small quanities of aluminium.

This is hobby only and I want to make it as simple and cost effective as possible for my first build to get good ideas and improve if necessary.

I am happy I am able to build the frame work but need to know about Hardware and Software

OK Questions,

1. If I was looking to purchase some cheap stepper motors from ebay (UK) what would I be looking for?

2. How would I match these motors to the controller and what controller would I be looking for? (how would this connect to my PC)

3. For a novice in software could anyone recommend anything?. Is there any freeware around?

4. Lastly, there are a lot of DIY builds on you tube and i'm quite impressed. I could'nt find any DIY designs on this site, are there any?.

Many thanks to everyone in advance, this site looks pretty amazing and I'm quite excited about starting a project.

Thanks Rob

p.s My apologies if this is in the wrong section of the forums.

irving2008
15-10-2008, 11:44 PM
Hello all,

I only thought about building a mini cnc last night and have only spent about an hour looking around the net at ideas.

I did'nt really get anywhere but have a few ideas and thought i'd post a few questions to see if what I want to achieve is even possible.

I'm looking at building quite a small CNC to enable me to machine materials such as: hard resin, nylon and possible small quanities of aluminium.

This is hobby only and I want to make it as simple and cost effective as possible for my first build to get good ideas and improve if necessary.

I am happy I am able to build the frame work but need to know about Hardware and Software

OK Questions,

1. If I was looking to purchase some cheap stepper motors from ebay (UK) what would I be looking for?

2. How would I match these motors to the controller and what controller would I be looking for? (how would this connect to my PC)

3. For a novice in software could anyone recommend anything?. Is there any freeware around?

4. Lastly, there are a lot of DIY builds on you tube and i'm quite impressed. I could'nt find any DIY designs on this site, are there any?.

Many thanks to everyone in advance, this site looks pretty amazing and I'm quite excited about starting a project.

Thanks Rob

p.s My apologies if this is in the wrong section of the forums.

Rob,

Welcome to MYCNCUK :)

I'll try and answer your questions, but as I said on CNCZone there is no right answer until you have some idea of the size/shape/performance of the machine you plan to build. For example are you going to use ACME/Trapezoidal screws with a delrin nut or a ballscrew? The difference is friction coefficient is 0.8 to 0.2 roughly - that could make a big difference to the torque required from the stepper motor. Similarly the pitch of the screw and the traverse rate you plan will determine the maximum speed you require from the motor which, depending on the 'knee' of the motor torque curve will determine the voltage you need to run. The torque and therefore the peak current, the voltage and the minimum step size will determine the characteristics of the driver, and whether it microsteps or not which ultimately determines the cost.

As far as motor sizes, anything less than a Nema23 frame motor is likely to be too small. Similarly anything less than a 200 step motor is unlikely to give the resolution required. This rules out most of the very cheap motors on eBay. Something like this (http://cgi.ebay.co.uk/3Nm-23-stepper-motor-New-with-12-Month-Warrantee_W0QQitemZ110277724627QQcmdZViewItem?hash =item110277724627) would probably do, but that 4.2A rating is going to mean a big, expensive driver. In simple terms a motor up to 2A can be driven with a cheap (30 - 50 per axis) driver which will give 1/2 and full step capability, above that you need to go to a more sophisticated driver which costs more but will give 1/4, 1/8 and maybe 1/16 step. Somthing like this (http://cgi.ebay.co.uk/1-86-Nm-stepper-motor-New-12-months-warrantee_W0QQitemZ110277723325QQcmdZViewItem?hash =item110277723325) would also suit, its 1/2 the torque of the bigger motor.

Motion Control (the firm advertising those motor) can supply matching drivers and power supplies. They, like many other suppliers (I am not recommending them specifically) can advise you to some extent but you need to have an idea of your requirement first. Too often we see "I've bought this set of motors, etc., will they do for my mill" only to have to advise they've not got the power, etc. to do the job. Too big can be as bad as too small.. there are resonance effects which come into play and a big motor run below its optimum settings can give as poor results as too small a motor.

The one thing you must have is a good quality breakout board that connects your PC's parallel port to the stepper drivers. An optically isolated board is best as it protects the PC against nasty voltages and resolves any ground loop issues.

Software: The two main products to drive the steppers from the g-code file that describes the motion required are Mach3 for Windows or EMC2 for Linux. Both are good, Mach3 isn't free but the freeware version can handle up to 100 lines of G-Code which is enough for many small jobs. In either case a dedicated PC with the minimum software on it is really essential, especially the Windows approach.

To get g-code you need to generate CAD drawings and process the output files, usually in DXF (autocad) format. I use QCad as my CAD package on Linux as its free if you compile it from source code. There are others. For small requirements freeware such as GDraw can do the CAD and g-code generation in the same package but are fairly limited.

Browse the Open Source Software forum on CNCZone for more ideas.

purple_rob
18-10-2008, 09:56 AM
Many thanks for your input. Has made me think about what I actually want.

I would really like to start cheap and small. Mainly because I want to learn the build and software and do not have anything major I want to build.

Lets say I wanted to machine a relativly simple engine block from a hard resin about 50mm by 30mm.

What I need is a simple kit or list of parts, like a small DIY kit. I saw this

http://cgi.ebay.co.uk/3-AXIS-CNC-Router-or-Mill-Stepper-Motor-complete-kit_W0QQitemZ160292226637QQcmdZViewItem?hash=item1 60292226637&_trkparms=72%3A1301%7C39%3A1%7C66%3A2%7C65%3A12%7C 240%3A1318&_trksid=p3286.c0.m14

but its way out of my price range and too big.

I have a friend who works in an alloy supply company and has all kinds of tubes there. I was thinking about usings maybe alloy beams and roller bearings for the build. Its something I can get and already have.

Unless I can ebay or machine some cheap parts. I do have some old printer inside that has some really nice steel rods.

Any advance on the ideas :)

irving2008
18-10-2008, 08:50 PM
Rob,

Don't forget if you order something from outside EU you will probably have to pay VAT and import tax (8% on electronics I think) so that puts the price up (even if you think you've got away with it they find you months later as I found out once :( )

"Cheap and small" and CNC in the same sentence is an oxymoron! So far my 250 x 250 build has cost me 40 in MDF, 55 for the steppers, 180 for the rails & screws, 65 for the electronics and power supply (parts only, I'll make the PCBs), 20 for a box for the electronics, 40 for a 500W Dewalt die-grinder as a milling spindle, 30 for couplers and bearings and 20 for screws, bolts, etc. Add another 50-odd for tools and petrol... so I'm on the wrong side of 500 already...

Parts from a printer might do for milling PCBs or balsa wood but aren't robust for anything heavier... the milling spindle I have is 1.7kg and the likely cutting force I've designed for is 20N using a single flute 3mm cutter at 20,000rpm and a cutting speed of 1m/min. To get close to 0.05mm accuracy needs 20mm rails on the Y axis to avoid excessive deflection and 16mm rails on the X axis to support a maximum 4kg work piece. That's based on a 18mm MDF design with some additional steel angle reinforcement on the fixed gantry sides.

Again, think about what you plan to machine the resin with and what sort of accuracy you want. Hard materials need cutting power. A simple 130W Dremel or Proxxon hand-held rotary tool has a lot of run-out and so accuracy is poor and doesn't have the grunt for hard plastic or ali. A full-size 1500W router is robust but heavy. The 1000W Kress-type milling spindles are great but pricey.

An alloy tube with v-mounted skate bearings could work as a cheap linear motion rail, the bearings are cheap enough at 3 for 8, and heavier alloy tube as structural parts would do although box section is easier to work with.

hope that helps a little more...

proddy
03-08-2009, 08:49 PM
hi rob, like you im gonna start a mini build of my own, i have a good sense of what my m/c will look like but i aint got a clue about the hardware or software side of it, (pretty sad when i've been a cnc miller for 8yrs now).

i was watchin youtube an seen the cnc router carving a tigers head an that was it, i was hooked on 3d cutting, its been my life long dream for the last 2 weeks now lol

anyways, wish you all the best an make sure you let us know your progress

Swarfing
04-08-2009, 12:08 AM
Rob

In between doing my main build i started another the whole footprint of the machine is 400 x 400 x 350. Just need to finish off the Z, its using old printer steppers and will be controlled by DIY drivers. Its all steel frame 25 x 50 box section. As soon as i get a chance i will put some picks and details up on a log. Shame is i will have this finished before the main. The reason i did it was to try things out, its the best way to learn and it cost me almost nowt. Sometimes you need to just jump in - but before this comment bites me back as i did say on a previous thread to plan first???? sometimes it is acceptable. Join freecycle for your area and pick up a few old plotters and printers (steppers, 8,10 and 12mm spindles) you will be surprised how far you can go. Steel rule and a sharp pencil is your best friend. Remember to clamp everything before drilling and you won't go far wrong.......Mine may never right but i will never be right without experience and that has to cost somewhere. Good luck:yahoo: